Oh, OP, I feel for you and can hear the disappointment coming through. The excited talk about a “family Christmas” when you won’t even be there - it’s insensitive at best.
Of course your parents are free to go wherever/whenever they choose. But you’re allowed to feel sad about it and feel like they are, indeed, “choosing” your brother over you - the rational, adult part of your brain sees the facts, but the inner child feels the hurt.
If you look back and reflect, has your brother been the favourite? Is he the “golden balls” son? The fact that he’s been paying less rent to your parents than you have, because they see his saving for travel as a priority and more important than your saving for a house, suggests so.
£600 per month is a big chunk of your income - if they’re so comfortable then I don’t know why they take so much from you (I wouldn’t take anything from you in this situation, if you were my DD). It’s their choice, obviously, but I really think you should perhaps explore other options.
I know you’re not keen on a house share, but it feels like it might be a better option for you - put a bit of healthy distance between you and your parents, feel more independent etc? You’re still very young and should feel like you have the world at your feet, with opportunities to try different things, rather than saving every penny for a house deposit.
Perhaps have a think if there’s anything you fancy trying, because now it probably the best time, while you’re young, healthy and have no commitments. Travel/work abroad, learn a new skill or take a qualification in something just because it’s interesting, buy a narrowboat to live on, whatever?
Don’t limit yourself, is why I’m trying to say. But if the only thing you REALLY want to do is buy a house, then it sounds like you’re going about it then right way and doing pretty well. I know you’re working full time, but would you have any capacity to try a couple of evening shifts in a pub, or similar? (I did it when younger). The extra money would boost your savings and there’s a social aspect.
Regarding Xmas, well, it’s often a highly emotional and tricky time for people. It’s great that you had lovely Xmases in times past, but things change. Life is constant change,
and the only way to survive is to adapt (I know it sounds dramatic, but I’m getting older and have seen some ‘interesting times’). Like you, I had fabulous childhood Xmases, then grandparents died, then we had more lovely Xmases with my young DC, then parents died and my DD is almost grown. Things change, sad things happen, but we can still find enjoyment and happiness in different ways. The year I couldn’t face a huge tree, I bought a small real tree in a nice pot and decorated it solely with decorations I’d made/sewn and some tiny lights - I might do the same this year, as my lovely DM died last December and I don’t think Xmas will ever feel the same. I’d suggest you perhaps focus on making just one room nice and festive for yourself - little tree, pretty lights, whatever makes happy/cosy. You will feel the absence of your parents and brother, and it will be different, but that doesn’t mean it will be inherently bad or sad.
Whatever happens, you sound like a sensible, clever young woman and I wish you all the best with your saving and home-buying goal!